The present invention relates to an ATM cell forming device for an apparatus used on an ATM (Asynchronous Transfer Mode) communication network and, more particular, to an ATM cell forming device designed in consideration of congestion control over an AAL (ATM Adaptation Layer) frame buffer for storing AAL frames to be formed into cells.
A device used for a conventional ATM cell forming circuit to perform cell assembly processing for AAL frames, i.e., SAR (Segmentation and Reassembly) processing has been mainly formed on the assumption that it is used for an ATM cell forming circuit for a terminal apparatus connected to an ATM communication network. For this reason, this device is not based on the concept that ATM cells are output to different channels. Even if, therefore, the device is to be used for an apparatus on the input side of a switch or multiplexer, AAL frames to be formed into cells are stored in a queue buffer regardless of the output channels. The frames are then formed into cells in the order of storage in the queue buffer and output as ATM cells.
FIG. 1 shows an arrangement in which a conventional ATM cell forming circuit is used as a device on the input side of a switch. Referring to FIG. 1, a processor 101 performs a process corresponding to the instructions of software for controlling the overall device, and causes an SAR processing device 104 to perform cell formation processing for AAL frames through a processor bus 102. The ATM cells formed by the SAR processing device 104 are output to a switch 109. A memory 103 is accessed by both the processor 101 and the SAR processing device 104 to form ATM cells. FIG. 2 shows the arrangement of the memory 103.
The arrangement of the memory 103 will be described with reference to FIG. 2. The memory 103 is constituted by a frame cell connection information table 112 in which frame/cell connection information is stored, frame cell formation information 113 containing the header information of each cell, a frame buffer 115 in which information about each frame itself is stored, and a frame descriptor table 114 indicating the address information of each piece of information in the frame buffer 115.
The SAR processing device 104 is constituted by a connection information read section 105 which is connected to the processor 101 through the processor bus 102 and designed to queue the descriptors of frames in a frame descriptor queue 106 in a desired output order by referring to the frame cell connection information table 112 of the memory 103 in accordance with pieces of frame information received from the processor 101, a cell formation information read section 107 for reading out overhead information required for the formation of cells from frames from the frame cell formation information 113 of the memory 103, and an AAL processing section 108 for forming frames into 53-byte ATM cells and outputting them to the switch 109. The AAL processing section 108 has the functions of stopping a cell output operation and discarding a frame and a timer function to control congestion.
The input terminal of the switch 109 is connected to the output terminal of the SAR processing device 104 and designed to output an ATM cell to one of output channels (C.sub.0 to C.sub.n) 110 in accordance with the output channel address written in the header of the ATM cell output from the SAR processing device 104.
A back pressure signal generating section 111 for notifying congestion upon occurrence of congestion is arranged on the output side of the switch 109. The back pressure signal generating section 111 has the function of outputting a back pressure signal 111a to the AAL processing section 108 of the SAR processing device 104 through the switch 109 upon detection of congestion, the function of discarding cells upon occurrence of congestion, and the function of outputting a back pressure cancel signal to the AAL processing section 108 when congestion is canceled.
The operation of this conventional ATM cell circuit will be described next.
The processor 101 sets connection information in the frame cell connection information table 112 of the memory 103 in advance. When there is frame information to be output to the switch 109, the processor 101 writes the descriptor in the frame descriptor table 114, and the data of the frame itself in the frame buffer 115. Thereafter, the processor 101 transfers the descriptor to the SAR processing device 104.
The transferred descriptor is transferred to the AAL processing section 108 through the connection information read section 105, the frame descriptor queue 106, and the cell formation information read section 107 in the SAR processing device 104.
The AAL processing section 108 reads out 48-byte data as the payloads of ATM cells from the address indicated by the descriptor in the frame buffer 115, assembles the data into 53-byte ATM cells together with 5-byte cell headers, and outputs the cells to the switch 109.
When channel congestion occurs on the output channels for the switch 109, the back pressure signal generating section 111 outputs the back pressure signal 111a to notify the AAL processing section 108 of the occurrence of congestion, and at the same time, discards the transferred cells. At this time, the AAL processing section 108 initializes and restarts the timer. After the cell of the currently output frame are completely output, the AAL processing section 108 stops outputting the next frame.
When the congestion is canceled, the back pressure signal generating section 111 notifies the AAL processing section 108 of the end of congestion. With this operation, the AAL processing section 108 resumes the cell assembly and output operations. If the timer time exceeds a predetermined time before this notification of the end of congestion, the frames queued in the frame descriptor queue 106 are discarded altogether.
When a device used for such a conventional ATM cell forming circuit to perform SAR processing for AAL frames is used for an apparatus on the input side of a switch or multiplexer, even if a specific output channel for the switch or multiplexer undergoes congestion, and the corresponding congestion notification is returned to the SAR processing device, either the congestion notification is neglected or the output operation for all the cells is stopped.
In the method of neglecting the congestion notification, since the degree of congestion in the output channels increases, more cells must be discarded. In the method of stopping the transmission of all cells, even if only a specific output channel is in a congested state, and ATM cells can be output to another output channel which is not in a congested state, since the transmission of all ATM cells is stopped, the frame buffer undergoes congestion. As a result, the overall throughput of the switch decreases.